1. Technical Field
The present invention relates to a method and an arrangement for supplementing the pulsed gas metal arc welding process with continuous electrode feeding and material transfer from the electrode to the workpiece essentially in the form of one droplet per pulse without the occurrence of a short circuit, with the object of making vertical welding of V-joints easier, particularly in thick material, especially in aluminium or stainless steel, with improved joint quality and increased productivity. The method consists of causing the welding process to alternate between such pulsed welding and short arc or spray arc welding during a continuous welding run. The apparatus comprises a welding set that supports such a method, which method is called SuperPulse in the following.
2. Background Art
In gas metal arc welding with melting, continually-fed electrode, usually called MIG/MAG welding, the workpiece is heated primarily by the arc. The electrode is heated partly by the welding current flowing through the electrode tip, that is the free end of the electrode between the contact nozzle, where the current transmission to the electrode takes place, and the arc, and partly by the arc itself. The basic control of the welding process consists of obtaining a consumption rate for the electrode that corresponds to the rate of feeding the electrode forward. Additional objects of the control can, for example, be to control the quantity of heat transferred to the workpiece or to minimize spatter.
MIG/MAG welding is carried out in one of three modes. In short arc welding, the material transfer from electrode to workpiece is carried out by large droplets with the occurrence of a short circuit, as shown in outline in FIG. 2. As the process consists of alternating arc and droplet transitions with the occurrence of a short circuit, the average voltage between electrode and workpiece is low and accordingly the heat transmission to the basic material is moderate. When the supplied power is increased, the globular mode is entered, where the material transfer is carried out by a combination of droplets with and without the occurrence of a short circuit. The result is an unstable arc with a lot of spatter and welding fume. Welding in this mode is normally avoided. When sufficiently high power is supplied, the process enters the spray mode, where the material transfer is carried out with small finely-distributed droplets without the occurrence of a short circuit, as shown in FIG. 3. The amount of spatter is clearly lower than with short arc welding. The transmission of heat to the base material is greater and the method is primarily suitable for thicker welded materials. The short arc welding mode and spray arc welding mode are normally controlled by the same type of weld process regulator. The mode that is adopted by the process is determined by the parameters that have been entered in the process regulator.
The third mode is called pulsed welding and involves the use of a considerably more complex process regulator that also actively controls the actual separation of the droplets using a suitable current pulse. Each pulse separates off one droplet and the droplets are sufficiently small for a short circuit not to occur. This method, often called synergic pulsing, has the advantages of the spray mode in the form of little spatter without the disadvantages associated with the great heat transmission.
Pulsed welding has become a very common welding method nowadays with modern rapidly-controlled inverter power sources. A problem with pulsed welding is the requirement for very precise parameter settings. This problem has been partially addressed in recent years by the introduction of double pulsing or pulse-on-pulse capabilities in these power sources. This involves the introduction of a slower pulsing between two different short pulse parameter settings, in addition to the pulsing that has the object of separation of droplets (short pulsing). In this way, a slightly greater tolerance is achieved with regard to the sensitivity of the parameters.
A remaining problem has been welding vertical V-joints in thicker materials, for example, 5-10 mm thick. Great skill has been required in order to weld such joints with acceptable quality. This has applied in particular when welding in aluminium or stainless steel. In order to bridge gaps when welding the root run as well as in order to obtain sufficient penetration and to avoid the weld pool running downwards during welding of sealing runs, the welder has been forced to use a weaving motion and, in this way, control the heat input at any time. This is both tiring and time-consuming and, even with this, the back of the welded joint has usually been concave, which results in poorer strength than if the back had been a convex shape. In order to achieve the required convex shape, some form of backing bar has often had to be used.